This invention relates to semiconductor packages in general. More particularly, the flow of an adhesive sealant is controlled by stamping a chamfer in the peripheral edge of an electronic package component.
Adhesively sealed electronic packages designed to encapsulate one or more semiconductor devices are generally described in U.S. Pat. No. 4,939,316 to Mahulikar et al which is incorporated by reference in its entirety herein. These packages have separate base and cover components which define a central cavity for encasing the semiconductor devices. A leadframe is disposed between the base and the cover and adhesively bonded to both. Small diameter bond wires electrically interconnect semiconductor devices to the inner lead ends of the leadframe. The outer lead ends of the leadframe are electrically interconnected to external circuitry such as a printed circuit board.
The electronic package base and cover components can be formed from any suitable material such as a metal, plastic or ceramic. To maximize the dissipation of heat generated by the semiconductor device during operation, the base and cover components are typically formed from a metal or metal alloy such as aluminum or an aluminum based alloy.
The adhesive used to join the base and cover components to the leadframe is typically a thermosetting epoxy. An epoxy preform (a windowframe shape sheet of epoxy) having an outside peripheral dimension approximately matching the outside peripheral dimension of the package is disposed between the leadframe and the base. A second epoxy preform is disposed between the leadframe and the cover. The assembly is then heated, usually to a temperature of about 180.degree. C. and the epoxy bonds the components together. At the seal temperature, the viscosity of the epoxy is reduced and the epoxy readily flows.
The epoxy flow must be controlled. If the epoxy flows into the central cavity, it can interfere with the bond wires. If the epoxy extends beyond the perimeter of the package, commonly referred to as squeeze-out, the epoxy can interfere with forming (bending) of the outer leads such that the leads will not have the required shape or location. Excessive squeeze out can jam or break the trim and form equipment or can lead to the assembled electronic package failing to fit in a test socket or a shipping tray.
Process adjustments to control squeeze-out, such as rigorous control of the thermal profile of the epoxy cure cycle, thickness of the package components, thickness of the epoxy preform, thickness of the assembled package, location of the package in the oven during epoxy cure and tilting of the oven shelves during epoxy cure have not proven effective to eliminate squeeze-out.
U.S. Pat. No. 4,262,165 to Ohwaki et al. discloses that to prevent a sealing glass from extending beyond the perimeter of an electronic package, the sealing glass is screened so that a narrow peripheral portion of the seal area remains free of the sealing glass. During package sealing, the sealing glass flows into this peripheral portion, but does not extend beyond the package perimeter. This solution is suitable for a sealing glass which has a relatively high viscosity, but is not effective for adhesives having a relatively low viscosity at the seal temperature and relatively uncontrolled flow.
U.S. Pat. No. 5,239,131 by Hoffman et al. and which is incorporated by reference in its entirety herein discloses forming a moat between the central region of an electronic package and the inner leads of the leadframe. Excess adhesive is contained by the moat and does not bridge to the inner leads.
There remains, however, a need for an effective means to control the adhesive squeeze-out in electronic packages.